Integral motor-tachometer system



Jan. 25, 1966 w 3,231,807

INTEGRAL MOTOR-TACHOMETER SYSTEM Filed Dec. 18, 1962 2 Sheets-Sheet lSTABLE PP0WC0 FREQUENCY REFERENCE CLOCKPaLSA-S SOURCE \CLOCKTQACA/PVCSESMAG/V5776 '9 P2000650 cx p r (10 M555 AMPLIFIER PHASE COMPARATOR DATASOURCES acszevo/wame wlr /wrzez/m 2| Ma a/145715? I0 I MOTOR DRIVE l4AMPLIFIER no. POWER SUPPLY max K W/u/s INVENTOR.

:EI|3 a BY f ATTOPAEY Jan. 25, 1966 J. K. wu |s 3,231,807

INTEGRAL MOTOR-TACHOMETER SYSTEM Filed Dec. 18, 1962 2 Sheets-Sheet 2aw/Al FIE 2I INVENTOR.

ATTORA/EY United tates atent C) 3,231,807 I INTEGRAL MOTOR-TACHOMETERSYSTEM Jack K. Wiliis, San Bruno, Calif., assignor to Ampex Corporation,Redwood City, Calif., a corporation of California Filed Dec. 18-, 196-2,S'er. No. 245,582 6 Claims. (Ci. 318313) This invention relates tosystems for sensing or controlling motor speeds, and more particularlyto systems for achieving extremely precise control of magnetic tapedrives.

Many electromechanical systems are known which operate to sense thespeed of a rotating member and which generate a control signalrepresentative of the speed of the member, which control signal may befed back to maintain the speed at a desired rate or within desiredlimits. Many tachometer generators and servo systems are known for thispurpose, and various other error signal generators will also suggestthemselves to those skilled in the art.

Known speed-control systems of this nature for governing the speed ofrotating drive members operate with moderate degrees of precision. Anumber of modern systems, however, require degrees of precision whichfar exceed the levels obtainable with most current systems. Modernmagnetic tape systems, for example, are used for accurately recordingand reproducing analog or digital signal data. In order to achieve highinformation rates and provide wideband recordings, such systems mustoperate with high relative head to tape speeds. Thus it is common, whenrecording a frequency band covering one megacycle, to use tape speeds of120 inches per second. Similarly, in recording digital data with highdensity; speeds of 150 inches per second are common. High densityrecording techniques, moreover, demand highly precise speed control forvirtually all conditions of operation, because minute mechanicaldisplacements result in the introduction of very great variations intoelectrical signals. At all times during both recording and reproducing,therefore, the speed of the tape and the driving member must bemonitored and corrected with a high degree of precision.

Such requirements create a need for controlled speed driving mechanismswhich are substantially free of mechanical errors which might otherwisebe tolerable. Errors o'f micro-inches must now be considered assignificant because of the disruption which they can introduce intoanalog or digital signal information. Errors of this magnitude are foundin all mechanical coupling arrangements, and are introduced by manydifferent dynamic effects. A tachometer which is coupled by a gear driveto a motor, for example, would be subject to err-or in representing theinstantaneous position and the speed of the motor because of mechanicalslippage and frictional effects. Moreover, the tachometer would beunlikely to be able to provide an adequately fine resolution ofrotational position. Furthermore, such errors as those introduced bytorsion in the shaft between the rotor of the motor and the tachometerduring rotation become significant in these applications.

It is therefore an object of the present invention to provide improvedand extremely precise controlled electromechanical driving systems.

A further objectof the present invention is to provide an improvedsystem for determining rotational position and speed of a drivingmember.

A further object of the present invention is to provide an improvedtachometer system for a motor drive for modern magnetic tape systems.

Systems in accordance with the present invention utilize an integralcombination of a readily controllable motor 3,231,807 Patented Jan. 25,1966 and a lightweight, high resolution signal generator forming anintegral part of the motor. An indicia bearing member peripherallydisposed about the rotor of the motor and having sensing elementscontained within the motor housing provides an exact representation ofinstantaneous rotor position without materially increasing the moment ofinertia, and without introducing any drag in the motor movement.

In a preferred form of system in accordance with the invention, therotor is disposed as a relatively thin disc rotatable on a central shaftand having windings etched or deposited on the flat surface of the rotorand in energy interchange relation with magnetic pole members. The outerperiph ery of the rotor terminates in a thin annulus which is affixedthereto and which includes a phot-ographic'ally ailixed for example)index pattern having extremely fine incremental indicia. Photoelectricsensing means are disposed within the motor housing adjacent the indiciaring. During rotation of the rotor, the photoelectric sensing meanscontinually generates a high frequency signal, the instantaneousfrequency of which is substantially representative of the instantaneousspeed of rotation of the rotor. Co'ncur-r'ently, the rotor design isparticularly suitable for operation at highly changing rates ofacceleration and frequency.

A better understanding of the invention may be had 'by reference to thefollowing description, taken in conjunction with the accompanyingdrawings in which:

FIGURE 1 is a block diagram and partial perspective representation ofthe principal elements of a drive control system in accordance with theinvention.

FIGURE 2 is a perspective view of a motor arrangement in accordance withthe invention with a portion of the motor casing removed to show theinternal structural elements;

FIGURE 3 is a side sectional view of an arrangement in accordance withthe invention;

FIGURE 4 is an enlarged fragmentary view of a portion of a disc rotorand photoelectric indexing means as employed in the mechanism of FIGURES2 and 3;

FIGURE 5 is an enlarged fragmentary view of a portion of a disc rotorillustrating an index pattern formed by deposition processes; and

FIGURE 6 is a greatly enlarged fragmentaryview illustrating a portion ofthe disc rotor and a reflective photoelectric indexing arrangement.

The present invention may be utilized in both open and closed loopcontrol systems. A leading example of application of the invention isfound in the art of magnetic tape transports, and accordingly theinvention is described in that context.

In a typical magnetic tape instrumentation recorder, the" principalelements of which are shown generally in FIGURE 1, a magnetic tape 10 istransported by the action of a drive capstan 11 and pinch roller 12between a pair of reels 13 and 14, one of which acts as a supply reeland the other of which serves as a tal'ceup reel for the principaldirection of tape advance. The tape may be supported by guide and idlermechanisms (not shown) so as to be constrained to move in a precise pathadjacent the magnetic head system 15. For simplicity, a single headarrangement 15 is shown, although it will be appreciated that systems ofthis type usually have multiple parallel heads and may employ separateheads for recording, reproducing and erasing.

It is assumed that the system of FIGURE 1 is a wide- :band recordingsystem, and that the tape 10 is to be driven with a minimum of speedvariation at a selected rate of speed. The rate may be high or low,depending upon the bandwidth it is desired to encompass. During thisrecording, conventional techniques may be used to add a synchronizingtrack, to which reference may be purposes.

made during reproduction in order to precisely recreate the originaltime base. Accurate sensing and control of the speed of a motor for thedrive capstan 11 is extremely useful in a number of differentapplications. If the carpstan speed can be held to very close tolerancesduring recording, and during reproduction, a minimum of additional speedcorrection circuitry will be needed to estab lished a precise time base.This can be done by electronically comparing the frequency of the signaldrive, as by sensing the motor speed, to an ultra-stable frequencysignal, and making appropriate speed corrections, either by a closed oropen loop system.

The same precise time base stability also becomes useful in digitalrecording systems. Here, however, movement of the tape is essentiallydiscontinuous, in contrast to the essentially continuous tape movementwhich is used in analog and instrumentation recorders. Minimization ofstart and stop times and distances is essential to most digital datatape transports. The capability of accurately monitoring speed withoutintroducing an added inertial factor not only permits reduction of themargin which must be related to start and stop times, but permitsaccurate analysis and improvement of start and stop transients andcharacteristics.

In the system of FIGURE 1, a closed loop magnetic tape control system isshown for operation of the tape system in the recording mode. A capstandrive motor 16 is coupled to drive the capstan 11 and the tape at veryclosely controlled rates of speed in this mode. It is desired that speedbe held within much closer limits than is feasible with a conventionalsynchronous motor, for example, one operated from a 60 c.p.-s. supply.Accordingly, the speed of rotation of the motor 16 should be sampled ata high rate, for comparison with signals from a highly stable frequencyreference source 17 and used for generation of a suitable error signalfor control In systems in accordance with the present invention, aphotoelectric signal generating system including a member coupleddirectly to the rotating member of the capstan motor 16 generates anextremely high frequency signal. This signal represents, by variationsin frequency, the variations in motor speed. In another sense, itprovides an extremely fine resolution of the rotational position of therotor.

The signal components which are representative of actual speed ofrotation of the capstan drive motor 16 are amplified by amplifier 18 andcompared to the reference signals in a phase comparator 19, and theerror signal generated thereby is applied through a conventional servoamplifier 21 as a correctional signal. Concurrently, the referencesignal may also be recorded on the tape 10 to provide a reference whichmay be utilized during operation in the reproduction mode.

- Stationary magnetic pole elements 35 facing but spaced apart from therotor disc 32 provide magnetic flux patterns which are intersected bythe rotor windings 34.

. Contact elements (not shown) coupled to a D.C. supply and in operativeengagement with the selected areal segments of the rotor circuitsprovide driving power and the needed energy interchange with themagnetic fields to generate rotational motion. The D.C. magnetic fieldsare of extremely high density and the energy interchange relationbetween the rotor and stator fields is extremely efficient, but theinertia and mass of rotor 32 are relatively very low. This means thatthe rotor 32 can be accelerated and decelerated rapidly, and that veryrapid speed changes can be effected even at high rotational speeds. Theeificiency of energy interchange can be further enhanced by provision ofa magnetic backing material 36 provided on the opposite face of therotor disc 32 to complete the magnetic field on the opposite side of thewinding patterns 34. Accordingly, variations of drive current can beused to achieve motor responses as high as 3 or 4 kilocycles persecond-Without difliculty. The motor 30 can therefore theoretically beheld in synchroni-sm with a reference signal at this frequency, althoughthe reference would necessarily have to be at a considerably higherfrequency for full utilization of the motor response capability. Thetheory of operation of similar printed circuit motors is more fullyexplained by reference to U.S. Patent No. 2,970,238.

The indexing pattern 31 at the outer periphery of the rotor 32, togetherwith its associated sensing device, provides the sought-for resolution.By preparation of a master having approximately 35,000 total incrementsabout a circumference corresponding to that of the outer rotor ring 37,a corresponding pattern may be introduced from this master onto theouter rotor ring 37 simply by contact printing. The desired indiciapattern 31 may be introduced on a master by such conventional techniquesas precision mechanical scribing on an expanded scale or electronicexposure of selected areas on a photographic plate by bombardment with afinely focused electron beam which is precisely positioned at differentradial positions to produce successive contrast regions. Whatevertechniques are used, a minimum of preparation of the rotor 32 itself isneeded once the master is prepared.

The indicia bearing outer ring member 37 may be a separate ring aflixedto the periphery of the rotor. It is preferred, however, to utilize afiberglass-reinforced, resin-impregnated rotor. This provides alightweight high strength structure which may include an indicia bearingmember as an integral part. The fiber glass reinforcement is terminatedshort of the indicia 31, so that the outer annular part 37 on which theindicia 31 are -placed is made only of a thintransparent synthetic resinmaterial. This may be prepared for the reception of the indexing pattern31 simply by an addition of a photosensitive layer which may be exposedunder the master and developed in the usual Way.

With a rotor of, for example, 5 inches in diameter, the outer annulus 37may be less tha one inch in radial extent, with a slight further spacingfrom associated rotor elements. This may readily be enclosed in themotor housing without special modification.

In addition to the insignificant mass and inertia provided, thisconstruction involves a photoelectric sensing system which leaves therotor completely free from all external drag. The circumferentialpattern divisions 31 are extremely fine, and for proper resolution ofthe patterns, emanations from a light source 41 are passed through acollimating device 42 so that the phototube 43 is sensitive only to avery narrow radial line of light having a width less than one of theindexing marks of the pattern 31. The collimating device 42 may consistof a series of collimating slits 44 suitably held in place on either orboth sides of the transparent outer annulus 37. Preamplifiers andassociated processing circuitry coupled to receive electrical signalsgenerated in the phototube 43 have not been shown in detail forsimplicity.

It should be recognized that various other photoelectric and electronbeam sensing systems may be employed with equal advantage with anappropriate indexing pattern. FIGURE 6 illustrates a reflectivearrangement wherein a light beam source 25 is positioned with acollimating device 46 to bounce a beam of light from an opaquereflective surface on the outer annulus 47 to be received by a phototube48. The indexing marks 49 in this case have contrasting reflectiveproperties, which in the case of the light beam would be darker areasfor absorbing the normally reflected light rays.

Under all conditions of operation, therefore, the resolution systemgenerates a periodic electrical signal from .which accurately resolvesthe instantaneous rotor speed "'19 is very sensitive to variationsinfrequency of the "source 17, -and-thus recording speed can? beheld-.very

the output of thephototube 43 or like recei ving electrode ji(iandfftherefore'thetape speed). :The' phase comparator iphototub 'sign-alfrom thestandard frequency of the stable closely to the desired time base. Evenat very low tape speed (a frequency divider cotipledto the stable source17 may be used with tape speed reductions unless reduc tions areeffected by ,mechanicaljigearing), the resolution of said. disc-likerotor; tachometer sensing means disposed within said'housing andadjacent said rotor for sensing said indicia means and for producing .asignal frequency proportional to the speed of the rotor;

is sufliciently fine totrv'intual'lyte im ate"swow'androthe reference.signal generat ng me ns p ovid ng sta l slow term-recording errors.Therefore, the t me Se reference frecnlency;v ror may be held Within arangeover which electronically means for comparing th pfgpprtiggglsignal frequency adjustable delay line techniques may fully compensatefor to the reference signal frequency; and the errors. means responsiveto said comparing means coupled lvtoderi i f .printed circuit etchingand deposition techt id motor for i i i id rotor at a pro niques aresuitable for providing the finely divided indexdeter in d speed, ingpattern 31 along with the electrical Winding pattern 4, A motor speedontrol y tem, comprising: 34 0n the? f A fragment 9 a rotor disc P p aprinted circuit motor including a transparent disc-like in this mnnneris illustrated detail in FIGURE 5 2 rotor having windings imprintedthereon, a shaft Here a a n? the light P ot he throughv the basemounting 'said rotor and coupled to a load for driving member (Outerrotor S g but the radial transsame, a stator element mounting said shaftand elecmissivity portions are providedby the minute metallic ti llydriving id ot d h ft, nd conductive segments 51. If the rotor housing isevacua h ing element enclosing id t t aid rotor ated of'air, of course,readout :of much finer gradations d -ti f id h ft; y be a'i qo by a eetelectron heath said disc-like rotor being provided with peripherallydis- Ieeted w r a target ode through the patternsposed and equallyspaced opaque sections composed AS is a -kn the focused electron beamcan also be of etched e-lectro-deposited electrically conducting usedin" reflective techniques for generation of signals. i l; This y t th beSimilar t the light reflective tech light source means mounted withinsaid housing ele- Iliqlle shown in FIGURE 6, wherein a collimated lightment and affixed to one of said elements for directing beam is replacedby a hoe electron beam reflected off a a collimated beam of light ofless width than any one small area on the indexing pattern toward ashielded f Said opaque Sections toward one Side f h target ttodeelement. riphery of said disc rotor to impinge alternately on Whilethere has been described particular embodiments Said opaque Sections d itransparent rotor i of a motor control system according to theinvention, rotation; i will bejhhderstood by those Skilled in the artthat the photosensitive means mounted within said housingeleforegoingand other changes inform and details may be ment and afiixodto one of Said elements on h made ther lnlwithout departing from thespirit and scope posite Side f the periphery of Said rotor for receivingof the ny9n 9 40 said beam after passage through said transparent rotlsclalmed t tor and for delivering an output signal of frequency A totoh e Control System compnslhgi proportional to the speed of rotationof said rotor; a motor tilheludlhg a Shaft and a rotor 1n the form of areference signal generating means providing a stable disc having a smallmoment of inertia mounted on reference'frequency; Said eh t means forcomparing the proportional signal frequency a load coupled to said shaftso as to be driven thereby; to the f rence signal frequency; and indiciameans circumferentially disposed about the disc means responsive tosaid'oompnring means d l d rotor and having varying trahsmisstvhyPatterns to said motor for maintaining said rotor at a preh I determinedspeed. sensing means disposed adjacent the indexing means A motor speedcontrol system, i i

f Sensing the transmissivity Patterns and for a printed circuit motorincluding a disc-like rotor havduethg a Signal q h y Proportional to theSpeed ing windings imprinted thereon, a shaft mounting f the rotor; saidrotor and coupled to a load for driving same, a reference Signalgenerating means Ptovlthhg a Stable stator element mounting said shaftand electromagfetefehee q y; netically driving said rotor and shaft, anda housing means for comparing the P po slghel frequency elementenclosing said stator, said rotor and a porto the reference signalfrequency; and n f Said Shaft; means responsive to Said comparing meansfor said disc-like rotor being provided with peripherally lusting theSpeed of the mototdisposed and alternating light reflecting and non- 2.A motor speed control system, comprising: fl ti Sections; a printedcircuit motor including a disc-like rotor havlight source means mountedWithin Said housing ing windings imprinted thereon and a shaft mountingment and affixed to one of said elements for directing a rotor andcoupled to a load for drtvlhg same; a collimated beam of light of lesswidth than any one tachometer indicia means formed on the periphery ofof said Sections toward one Side of h periphery f Said di 6 fOtOY; 5said disc rotor to impinge alternately on said reflecttachometer sensingmeans disposed adjacent said rotor ing sootions and Said nonnoiieotingsections during for sensing said indicia means and for producing arotation f Said rotor; Signal frequency Proportional to the speed of thephotosensitive means mounted within said housing eleand ment and affixedto one of said elements for receiving mea for receiving Said Signalfrequency coupled to 7 said beam after the reflection thereof from saidre- Said mOtOf for maintaining said rotor at a PTedeter' fleetingsections and for delivering an output signal of mined speed. frequencyproportional to the speed of rotation of 3. A motor speed controlsystem, comprising: said rotor; a printed circuit motor including adisc-like rotor havreference signal generating means providing a stableing windings imprinted thereon, a shaft mounting reference frequency;

3 23 1 807 7 3 means for comparing the proportional signal frequencyReferences Cited by the Examiner to the reference signal frequency; andUNITED STATES PATENTS means responsive to said comparing means andcoupled I to said motor for maintaining said rotor at a pre- 2,769,94911/ 1955 Stratum -7 1 determined speed 5 JOhl'lSOIl 3 6. A motor controlsystem, comprising: 3,016,428 1/ 9 a ll t al 318--314 X a printedcircuit motor including a disc-like rotor hav- 9, 77 1 1964- Manteuffel318 13;; ing windings imprinted thereon, and a shaft mounting said rotorand coupled to a load for driving same; FOREIGN PATENTS tachometerindicia means formed on the periphery 10 503,944 6/1951 Belgium. v if ofsaiddisc-like rotor; and r Y tachometer sensing means disposed adjacentsaid rotor ORIS L. RADER, Primary Examiner. for sensing said indiciameans and for producing a signal frequency proportional to the speed ofthe GORDONAWSMM Examine"- rotor. 15 v

1. A MOTOR SPEED CONTROL SYSTEM COMPRISING: A MOTOR INCLUDING A SHAFTAND A ROTOR IN THE FORM OF A DISC HAVING A SMALL MOMENT OF INERTIAMOUNTED ON SAID SHAFT; A LOAD COUPLED TO SAID SHAFT SO AS TO BE DRIVENTHEREBY; INDICIA MEANS CIRCUMFERENTIALLY DISPOSED ABOUT THE DISC ROTORAND HAVING VARYING TRANSMISSIVITY PATTERNS THEREON; SENSING MEANSDISPOSED ADJACENT THE INDEXING MEANS FOR SENSING THE TRANSMISSIVITYPATTERNS AND FOR PRODUCING A SIGNAL FREQUENCY PROPORTIONAL TO THE SPEEDOF THE ROTOR; REFERENCE SIGNAL GENERATING MEANS PROVIDING A STABLEREFERENCE FREQUENCY; MEANS FOR COMPARING THE PROPORTIONAL SIGNALFREQUENCY TO THE REFERENCE SIGNAL FREQUENCY; AND MEANS RESPONSIVE TOSAID COMPARING MEANS FOR ADJUSTING THE SPEED OF THE MOTOR.